1. Field of the Invention
The present invention relates to a color image forming process. More particularly, the present invention relates to a color image forming process which comprises processing a photographic element comprising a support having thereon a layer containing imagewise distributed silver with an aqueous alkaline solution containing a stannous ion in the presence of a dye to bleach the dye in an imagewise manner.
2. Description of the Prior Art
In a general process of forming color images, azomethine dyes or indoaniline dyes are formed by developing silver halide light-sensitive materials in the presence of couplers using a primary aromatic amine developing agent. Color development using silver halide is based on the process invented by L. D. Nannes and L. Godowsky in 1935. Various improvements have been made in the process since and in general the system has been employed worldwide in the photographic art.
Color development using a primary aromatic amine developing agent generally has the following disadvantages. (1) The dyes formed by the system have poor light resistance, heat resistance, and moisture resistance and, hence, the color images formed have a great tendency to fade with the passage of time, (2) a primary aromatic amine developing agent is toxic, for example, causing skin rashes and, thus, specific precautions are required in using this type of developing agent, and (3) since there is an equivalency relationship between the dye image and the oxidation product of the color developing agent, it is theoretically difficult to reduce the amount of the silver halide which takes part in the dye formation to an amount lower than the stoichiometrically required amount.
Conventional techniques for reducing the amount of silver halide in color photography can be classified into reducing the amount of silver halide to as low as level as possible and reducing the stoichiometrically required amount of the silver halide itself. In regard to the latter approach, the so-called two equivalent couplers capable of forming one molecule of dye with two molecules of silver halide have been developed. However, even using this technique, it is theoretically difficult to reduce the amount of silver salt in the light-sensitive materials to less than 1/2 of the amount of silver salt in light-sensitive materials containing couplers other than two equivalent couplers.
Another color photographic process presently employed is the silver-dye-bleach photographic process. This process is based on the color photographic process invented by Gaspar and, since azo dyes are used in the color process, the color images formed by the process generally have excellent light resistance, heat resistance and moisture resistance. A typical photographic element used for the silver-dye-bleach color photographic process has three silver halide photographic emulsion layers respectively sensitized to red, green and blue light, and having associated therewith, respectively, a bleachable cyan, magenta and yellow dye. Such a photographic element provides color photographic positive images through the following processing.
(1) The photographic element is imagewise exposed.
(2) The exposed photographic element is developed in a silver halide developer to form negative silver images.
(3) The photographic element is then processed in a dye bleach bath which oxidizes the silver images to a silver salt and concurrently decolorizes the associated dye pattern, and
(4) Finally the photographic element is fixed and washed to remove the residual silver salt, whereby dye images are obtained which are photographically the reverse of the initial silver images. The silver-dye-bleach process is generally described in, for example, U.S. Pat. Nos. 3,498,787 and 3,503,741, Canadian Pat. No. 790,533 and A. Meyer, "Some Features of the Silver-Dye-Bleach Process", The Journal of Photographic Science, Vol. 13, pp. 90-97 (1965).
In the silver-dye-bleach process described in U.S. Pat. No. 2,270,118, dye images are formed by processing dye-containing layers having silver images therein with an acid solution which decomposes the dyes in the areas where silver is present. The decomposition or destruction of the dye is accelerated by various "catalysts" such as phenazine. Also, the reaction in these dye bleach systems is believed to proceed on a stoichiometric basis (for example, U.S. Pat. No. 3,340,060 in Column 1, lines 18-21 suggests that 4 atoms of silver are required for decomposing one azo dye group). However, these silver-dye-bleach processes have the following disadvantages:
(1) Since a large amount of silver is required for bleaching the dyes, the photosensitive materials must contain a large amount of silver halide in the silver halide photographic emulsion layers.
(2) Since a strongly acidic processing solution, which is highly corrosive, is usually used in these processes, difficulties are encountered in maintenance of the processing apparatus and handling the processing solution.
Also, another silver-dye-bleach process is described in U.S. Pat. Nos. 1,517,049 and 2,184,022. According to the method dyes are decomposed imagewise using sodium hydrosulfite or stannous chloride as a reducing agent. In the process, however, the decomposition rate of the dye is extremely small and thus a long bleaching time is required as shown in Examples 1 and 2 below. In order to carry out processing in a short period of time, it is necessary to use a vat dye such as indigo which is easily decomposed. Therefore, dyes suitable for a photographic system cannot be freely used which is a disadvantage of the method.